A magnetic bearing-shaft assembly having a bearing to support the shaft in the event of failure of the magnetic bearing

Abstract

 An assembly in which a rotating shaft (10) is supported by a magnetic bearing (13) has a back-up bearing (18), to support the shaft (10) in the event of failure of the magnetic bearing (13), the back-up bearing (18) comprising a first layer of a dry bearing material (19), and with a surface opposite to a wear-resistant surface portion of a second layer (20). One layer (19,20) is provided on a stationary part (16) of the assembly, such as a housing, and the other layer is provided on the shaft (10). When the coefficient of friction between the first and second layers (19,20) is either relatively low; or relatively high; heat can be dissipated from the back-up bearing (18), respectively, at a low rate; or at a high rate. Heat dissipation may be enhanced by providing fins (26) in good thermal contact with at least one of the two layers (19,20) of the back-up bearing (18); and each such layer (19,20), advantageously, has a high thermal conductivity.

Description

[0001] This invention relates to an assembly in which a rotating shaft is to be supported by a magnetic bearing, but is to be supported by a back-up bearing in the event of failure of the magnetic bearing. A magnetic bearing may be provided for a shaft having high rotational energy, and it is especially important in such a case to provide a satisfactory back-up bearing for the the shaft.

[0002] The back-up bearing is required to be of a form to support the shaft in a satisfactory manner, and to enable the shaft to be brought to rest, after any failure in the operation of the associated magnetic bearing. Whilst the shaft is being brought to rest it is required that the shaft, any part of assembly rotating with the shaft, and the back-up bearing, do not become damaged, or worn.

[0003] The shaft is brought to rest by the rotational energy of the shaft being absorbed by, and then dissipated in the form of heat from, the back-up bearing; and/or by the rotational energy of the shaft being absorbed by the system driving the shaft doing work. If a substantial amount of the rotational energy of the shaft is to be absorbed by the back-up bearing, there is to be a relatively high coefficient of friction associated with the back-up bearing. Alternatively, if a substantial amount of the rotational energy of the shaft is to be absorbed by the system driving the shaft, there is to be a relatively low coefficient of friction associated with the back-up bearing.

[0004] If the shaft is to be brought to rest in a short period the rate of absorption of the rotational energy of the shaft by the back-up bearing, and/or by the drive system for the shaft, is required to be high.

[0005] It is known to have a back-up bearing with rolling elements, for example, the back-up bearing comprising a ball race, possibly without the provision of any form of lubrication. Any such construction for a back-up bearing having rolling elements has a low coefficient of friction associated therewith, say of the order of 0.001. The ball race may be required to run beyond its design speed limit after any failure of the magnetic bearing. Further, the ball race may have suffered damage due to rotation of the shaft causing fretting, before any failure of the magnetic bearing. Thus, after any failure of the magnetic bearing it is common for the ball race to fail during run down of the shaft, which is clearly undesirable, and causes the requirement that the ball race is to be replaced before the magnetic bearing can be used again.

[0006] Bearings are known each of which includes a layer of a dry bearing material. Such bearings usually fail by seizure of otherwise relatively rotating, constituent parts of the bearing, due to high temperatures being generated within the bearing. Such high temperatures are generated because the rate of absorption of energy thereby is greater than the rate of dissipation of heat therefrom. It will be understood that the rate of dissipation of heat varies with the temperature of the bearing, and the bearing fails when, with the temperature at the value at which the constituent parts of the bearing seize, by being mechanically welded together, the rate of absorption of energy is greater than the rate of dissipation of heat. For convenience, hereinafter in this specification, and the accompanying claims, the term dry bearing material is used to refer to a known composition of a dry layer suitable to be included in a bearing. In particular, the layer is required to provide a surface which is less likely, than otherwise would be the case, to become seized with, by becoming mechanically welded to, the surface of a co-operating part rotating relative to the layer.

[0007] Thus, it is an object of the present invention to provide a back-up bearing for an assembly in which a rotating shaft is to be supported by a magnetic bearing, the back-up bearing having a construction such that it has a reduced tendency to be damaged, or worn, either when the associated magnetic bearing is operating normally, or when the shaft is being brought to rest in the event of failure of the magnetic bearing, compared with known back-up bearings having rolling elements.

[0008] In accordance with the present invention an assembly in which a rotating shaft is to be supported by a magnetic bearing, includes a back-up bearing having a first cylindrically-shaped layer of a dry bearing material, and with a surface opposite to a wear-resistant surface portion of a second cylindrically-shaped layer of the back-up bearing, one such layer is provided on a cylindrical part of the shaft, and the other layer is provided on a stationary part of the assembly, in the operation of the assembly, the clearance between the opposing surfaces of the back-up bearing is arranged to be less than the clearance between the rotating shaft and the magnetic bearing.

[0009] Usually, the second layer of the back-up bearing is provided on the cylindrical part of the shaft; but if the shaft is not to rotate at a high speed, instead, the first layer may be provided on the cylindrical part of the shaft.

[0010] Such an assembly is advantageous in that the back-up bearing does not become damaged, or worn, inadvertently during the normal operation of the magnetic bearing.

[0011] Further, the back-up bearing easily can be arranged so that the back-up bearing, the shaft, and any part of the assembly rotating with the shaft, do not become damaged, or worn, whilst the shaft is being brought to rest after any failure of the magnetic bearing. In particular, the first and second layers of the back-up bearing are selected such that there is a required coefficient of friction therebetween.

[0012] The wear-resistant surface portion of the second layer is contiguous with a remaining portion of the second layer. The rate at which heat can be dissipated from the back-up bearing is determined partially by the coefficient of thermal conductivity of the remaining portion of the second layer. Conveniently, so that there is a high rate of dissipation of heat from the back-up bearing, the remaining portion of the second layer has a high coefficient of thermal conductivity, and comprises a heat sink.

[0013] Inevitably, the coefficient of friction associated with any form of back-up bearing in accordance with the present invention is at least one order of magnitude, and possibly two orders of magnitude, greater than that of any known form of back-up bearing including rolling elements.

[0014] Also for convenience in this specification, and the accompanying claims, reference is made to the coefficient of friction between the first and second layers of the back-up bearing being relatively low when it is less than 0.2 and reference is made to this coefficient of friction being relatively high when it is greater than 0.2.

[0015] If the heat can be dissipated at only a low rate from the back-up bearing, for the shaft and the back-up bearing not to become seized, or worn, the coefficient of friction between the first and second layers is required to be relatively low. Thus, the material of the first layer may comprise a dry lubricant, such as graphite, held within a matrix of a sintered powder, say, of tin bronze.

[0016] Alternatively, if the heat can be dissipated from the back-up bearing at a higher rate, the coefficient of friction between the first and second layers can be correspondingly higher than that referred to in the preceding paragraph, without the shaft and the back-up bearing becoming seized together, or becoming significantly worn. Hence, the first layer may comprise a textile material, say, of asbestos or glass fibre, impregnated with a phenolic resin.

[0017] The rate at which heat can be dissipated from the back-up bearing can be enhanced by the provision of fins in good thermal contact with the first layer, and/or the second layer, of the back-up bearing. Desirably, fins are mounted to rotate with the shaft. The, or each, layer of the back-up bearing in good thermal contact with the fins, advantageously, has a high coefficient of thermal conductivity.

[0018] Conveniently, the stationary part of the assembly, with a layer of the back-up bearing provided thereon, comprises a housing of the assembly.

[0019] When fins are provided in good thermal contact with at least one layer of the back-up bearing, the fins may be arranged to be external of a housing of the assembly, and irrespective of whether a layer of the back-up bearing is provided on the housing, or not.

[0020] The present invention will now be described by way of example with reference to the accompanying drawing, comprising a side elevation, partially sectioned, of one embodiment of an assembly in accordance with the present invention, showing a rotating shaft mounted within a magnetic bearing, and there being provided a back-up bearing.

[0021] As shown in the accompanying drawing, an assembly includes a rotating shaft 10 mounted within a conventional magnetic bearing, indicated generally at 12. The electromagnet of the magnetic bearing has two coils 13, and a ferromagnetic core 14.

[0022] Only one end of the shaft 10 is illustrated, and this end protrudes through an aperture 15 in a part of a housing 16 for the assembly, only this part of the housing being shown. Within the aperture 15 in the housing 16 is provided a back-up bearing for the assembly, the back-up bearing being indicated at 18.

[0023] In accordance with the present invention, the back-up bearing 18 has a cylindrically-shaped first layer 19 of a dry bearing material, secured to the housing 16 by being an interference fit within the aperture 15; and a cylindrically-shaped second layer 20, with a wear-resistant surface portion 21, and secured to the shaft 10 by being a press fit on a reduced diameter cylindrical part 22 of the shaft 10. The radially inner surface 23 of the layer 19 is opposite to the radially outer surface 24 provided by the wear-resistant surface portion 21 of the layer 20.

[0024] In one example of the assembly, and with the assembly in operation, the clearance between the magnetic bearing 12 and the shaft 10 is 0.5 millimetre; and the clearance between the opposing surfaces 23 and 24 of the back-up bearing 18 is 0.25 millimetre.

[0025] It is required that the shaft 10 is brought to rest after any failure in the operation of the magnetic bearing 12, and before the shaft, and any part of the assembly rotating with the shaft, becomes damaged, or worn. In particular, it is required that the shaft and the back-up bearing 18 do not become seized, or significantly worn, because of temperature rises within the back-up bearing, and of the shaft.

[0026] Heat is dissipated from the back-up bearing by conduction of the heat to the shaft 10; to any part of the assembly secured to the shaft; and to the stationary part 16 on which the layer 19 of the back-up bearing 18 is provided. Further, at least the layer 20 of the back-up bearing 18 also may comprise a heat sink, having a high coefficient of thermal conductivity. Usually the shaft 10 has a high coefficient of thermal conductivity.

[0027] Thus, if the arrangement is such that heat can be dissipated only at a low rate from the back-up bearing before the back-up bearing 18 and the shaft 10 become seized together, the coefficient of friction between the layers 19 and 20 has to be relatively low. Hence, the shaft 10 takes a long time to be brought to rest.

[0028] Alternatively, if the arrangement is such that heat can be dissipated from the back-up bearing at a high rate before the back-up bearing 18 and the shaft become seized together, the coefficient of friction between the layers 19 and 20 may be higher than for the arrangement described above. Consequently, the shaft 10 may take only a short time to be brought to rest. It is required to ensure that the rate of conversion of the energy of the rotating shaft to heat within the back-up bearing is less than the rate of dissipation of heat from the back-up bearing at temperatures below those at which seizure, or significant wear or damage, of the assembly parts occurs.

[0029] As shown in the drawing, the rate of dissipation of heat may be enhanced by the provision of fins 26 in good thermal contact with the layer 20 of the back-up bearing 18 provided on the shaft 10. It is especially advantageous if, as illustrated, the fins 26 rotate with the shaft 10; the layer 20 is of a material with a high coefficient of thermal conductivity, greater than 0.90 calories per sec. cm. ^oC; and the fins are external of the housing 16 of the assembly. In the illustrated arrangement, the fins 26 are shown clamped to be contiguous with the layer 20 by a locking member 28 secured to the end of the shaft 10.

[0030] In one particular assembly in accordance with the present invention the shaft is of steel. The layer 19 of the back­up bearing 18 is of a textile material, say, of asbestos or glass fibre, impregnated with a phenolic resin. The layer 20 of the back-up bearing 18 has a wear-resistant surface portion 21 of hard chromium plated onto the remaining portion 29 of the layer; and the remaining portion of the layer comprises an alloy of copper and chromium, having a Vickers hardness value of 130, and having a thermal conductivity of 0.95 calories per sec. cm 0C. There is a relatively high coefficient of friction between the layers 19 and 20. The fins 26 are of an alloy of copper and chromium, having sufficient strength to withstand the rotational stresses at high speed, and to which the fins are expected to be subjected.

[0031] Alternatively, the layer 19 of the back-up bearing 18 comprises a dry lubricant, such as graphite, or molybdenum disulphide, or tungsten disulphide, or polyethylenetetra­ fluoride, held by a matrix of a sintered powder, say, of tin bronze. There is a relatively low coefficient of friction between the layers 19 and 20.

[0032] Whether there is required to be a relatively low, or a relatively high, coefficient of friction between the layers 19 and 20, it is desirably that these layers have surface portions which are less likely to become seized together, by becoming mechanically welded to each other, at temperatures expected to be generated in the operation of the back-up bearing.

[0033] Modifications to the illustrated assembly are possible.

[0034] It is not essential that the layer of the back-up bearing having the wear-resistant surface portion is provided on the shaft, and, instead, this layer may be provided on a stationary part of the assembly, if the shaft is not to rotate at high speeds.

[0035] The stationary part of the assembly may not comprise a housing.

[0036] When fins are provided they may be in contact with both constituent layers of the back-up bearing; or they may be in contact only with the layer of the back-up bearing comprising dry bearing material.

[0037] At least some of the fins may not rotate with the shaft; and/or at least some of the fins may not be external of a housing; and/or the, or each, layer of the back-up bearing with which fins are in contact may not have a high coefficient of thermal conductivity.

Claims

1. An assembly in which a rotating shaft is to be supported by a magnetic bearing, and includes a back-up bearing, is characterised by the back-up bearing having a first cylindrically-shaped layer of a dry bearing material,and with a surface opposite to a wear-resistant surface portion of a second cylindrically-shaped layer of the back-up bearing, one such layer is provided on a cylindrical part of the shaft, and the other layer is provided on a stationary part of the assembly, in the operation of the assembly, the clearance between the opposing surfaces of the back-up bearing is arranged to be less than the clearance between the rotating shaft and the magnetic bearing.

2. An assembly as claimed in claim 1, characterised in that the second layer of the back-up bearing is provided on the cylindrical part of the shaft.

3. An assembly as claimed in claim 1, or claim 2, characterised in that the wear-resistant surface portion of the second layer of the back-up bearing is of hard chromium plated onto the remaining portion of the second layer.

4. An assembly as claimed in claim 1, or claim 2, or claim 3, characterised in that the second layer of the back-up bearing has a remaining portion, contiguous with the wear-resistant surface portion thereof, and the remaining portion is of a material with a high coefficient of thermal conductivity.

5. An assembly as claimed in claim 4, characterised in that the remaining portion of the second layer is of an alloy of chromium and copper.

6. An assembly as claimed in any one of the preceding claims, characterised in that the coefficient of friction between the first and second layers is relatively low.

7. An assembly as claimed in claim 6, characterised in that the first layer comprises a dry lubricant held within a matrix of a sintered powder.

8. An assembly as claimed in any one of claims 1 to 5, characterised in that the coefficient of friction between the first and second layers is relatively high.

9. An assembly as claimed in claim 8, characterised in that the first layer comprises a textile material impregnated with a phenolic resin.

10. An assembly as claimed in claim 8, or claim 9, characterised in that fins are provided in good thermal contact with the first layer, and/or the second layer, of the back-up bearing.

11. An assembly as claimed in claim 10, characterised in that fins are mounted to rotate with the shaft.

12. An assembly as claimed in claim 10, or claim 11, characterised in that the, or each, layer of the back-up bearing in good thermal contact with the fins has a high coefficient of thermal conductivity.

13. An assembly as claimed in any one of the preceding claims, characterised in that the stationary part of the assembly, with a layer of the back-up bearing provided thereon, comprises a housing of the assembly.

14. An assembly as claimed in any one of the preceding claims, characterised in that a housing is provided for the assembly, fins are provided in good thermal contact with at least one layer of the back-up bearing, and the fins are arranged to be external of the housing of the assembly.

Drawing